1. Field of the Invention
The present invention relates to a magnetic recording medium which achieves a high S/N ratio and a method for producing the same.
2. Description of the Related Art
As the recording density of magnetic recording/reproducing equipment has been increased year by year, it is highly desired to provide a magnetic recording medium having excellent recording and reproducing characteristics in a short wavelength range. Today, a coating type magnetic recording medium in which magnetic powder is coated on a substrate is mainly used, and its properties are being improved to satisfy the above desire. However, such improvement is nearing its limit.
One of magnetic recording media which can exceed this limit is a thin film magnetic recording medium. The thin film magnetic recording medium is produced by a vacuum deposition method, a sputtering method or a plating method and has excellent recording and reproducing characteristics in a short wavelength range. Examples of magnetic materials used in the thin film magnetic recording medium are Co, Co-Ni, Co-Ni-P, Co-O, Co-Ni-O, Co-Cr, Co-Ni-Cr and the like.
In the practical production of magnetic tapes, the vacuum deposition method is most suitable, and a deposition tape comprising a Co-Ni-O magnetic layer is practically used as a Hi-8 type video tape.
One embodiment of the production method of a deposition tape will be explained by making reference to FIG. 1, which schematically illustrates an inner structure of a vacuum deposition apparatus.
A substrate 1 made of, for example, a polymer film is unwound from a supply roll 4, travels around a peripheral surface of a cylindrical can 2 in a direction of an arrow 6, and is finally wound on a take-up roll 5.
A vapor 9 of a raw magnetic material 7 such as metal cobalt or a cobalt alloy is evaporated from an evaporation source 8 and deposited on the substrate 1 to form a magnetic layer on the substrate 1. As the evaporation source 8, an electron beam evaporation source is preferred, since it can evaporate a high melting point metal such as cobalt at a high evaporation rate.
Between the evaporation source 8 and the cylindrical can 2, there are provided shielding plates 3A and 3B, which prevent excessive deposition of the evaporated atoms on the substrate.
Oxygen gas is supplied in a vacuum chamber from a nozzle 10 during the vapor deposition of the magnetic material.
The Hi-8 type video tape which is now commercially sold is usually produced by the above described apparatus.
In the Co-O or Co-Ni-O magnetic layer formed by the above method, the formed magnetic material has a columnar structure, and an axis of easy magnetization slants at a certain angle from a line normal to a plane of the magnetic layer. That is, the axis of easy magnetization is not in the line normal line to the plane of the magnetic layer or in the plane of the magnetic layer, but instead slants from the normal line in a plane including an incident direction of the atom vapor onto the substrate. In the commercially sold Hi-8 type video tape, an axis of easy magnetization slants at about 70.degree. from the normal line of a magnetic layer in a plane including a longitudinal direction of the tape. The longitudinal direction of the tape corresponds to the running direction of the substrate in FIG. 1.
In the future, a magnetic recording/reproducing apparatus will have a smaller size and a larger capacity. To realize this, a linear recording density and a track density of the magnetic recording medium should be increased. Then, in the case of a magnetic tape, a high S/N ratio in particular in a short wavelength range should be increased.